Callback function when loading JSON

[dmeehan]

Can there be (or is there already) a way to introduce a callback when JSON data has started/finished loading. I would like to add a 'loading' indicator each time a new dataset is waiting to load into the map.

[JasonSanford]

To be clear, you want a loading event for the initial load of the layer or for every time more features are fetched? (Each pan and zoom)

[dmeehan]

My use case if for everytime more features are fetched.

[kbsali]

:+1: my use case would be to be able to adjust the symbology definition depending on the data loaded.

[kbsali]

@dmeehan @dzwarg 's PR #37 does the job actually